Currently powder or liquid coating of aluminum-alloy extrusions or iron section bars of considerable length is carried out in suitable automatic continuous treatment plants, in which the workpieces to be treated are moved in a horizontal direction while being suspended in a substantially vertical orientation.
Normally, in such treatment plants a treatment process is carried out which comprises the following sequence of operation steps:
hanging the bars from a chain or chains of a constantly moving overhead chain conveyor at a distance (pitch) between successive bars that varies depending upon the speed of movement of the overhead chain conveyor and the overall dimensions of the workpiece being treated;
pre-treating of the workpieces in a tunnel, where various treatment cycles are carried out in accordance with testing standards which may include, e.g. degreasing, a first washing, deoxidization, a second washing, chromatizing, a third washing, a fourth or final washing with demineralized water;
drying, normally by means of hot air ventilation;
powder or liquid coating in a suitable booth or booths;
final baking, normally by means of hot air ventilation; and
unloading of the coated workpieces.
In any case, pre-treatment according to one of its numerous variants in the tunnel is required in order to obtain optimum preparation of the metal surfaces for receiving and permanently holding the coating material thereon, so that a uniform and aesthetically attractive coating is obtained which is waterproof over time.
Actual pre-treatment steps (except washings) carried out in the tunnel involve the use of highly corrosive liquids, which must be kept mostly within preset temperature ranges for predetermined exposure time intervals to obtain optimum results.
Overall pre-treatment action is thus obtained through proper combination of the following parameters: exposure time, temperature, degree of corrosiveness of the liquids and amount of recycled liquid poured onto the surface being treated.
The most used types of pre-treatment are the following:
a) Static Dipping System
This is the oldest system. Pre-treatment liquids are kept almost in a static condition and thus the wetted surface of the workpiece does not come in contact with new liquid, which means that chemical degreasing or detergent action is limited and long exposure times and/or high concentrations of corrosive products are required.
b) Dipping System with Oscillatory Movement of the Workpieces
This dipping system is slightly more effective than the previous one since the workpiece movement, although to a limited extent, promotes renewal of the liquid that comes into contact with the surface of the workpieces. From a practical point of view, this system leads to results that are more or less equivalent to those achievable with the previous system.
c) Spraying System with Flight Manifolds and Nozzles
This is the most common system currently in use because it makes it possible to feed the workpieces and promotes almost continuous change of the liquid wetting the workpiece surface.
The latter system, however, has limitations and drawbacks that will be illustrated with reference to FIGS. 1 and 2 of the accompanying drawings, in which:
FIG. 1 shows a diagrammatic front sectional view of a pre-treatment spraying plant with flight manifolds and nozzles.
FIG. 1a is a diagram showing the amount of liquid flowing down along a workpiece, whereas
FIG. 2 shows a top view of the plant shown in FIG. 1.
With reference to a pre-treatment tunnel as shown in the above listed Figures, it will be easily noted that each jet G from each of the nozzles U supported by the lateral flight manifolds R affects only a section or length portion of a workpiece P. For this reason, workpieces P do not come in contact with the same amount of sprayed liquid. The amount of liquid running down along each workpiece P being fed throughout the spraying stage increases from top downwards, e.g. as shown in the diagram (amount of liquid A/height B) in FIG. 1a. 
Thus, each workpiece P is better treated at its lower portion than at its upper portion.
It has already been suggested that this problem can partly be solved by distributing the nozzles U at a non-uniform distance from each other, i.e. a lesser distance at the upper portion and a greater one at its lower portion so as to better balance the distribution of sprayed liquid. This expedient, however, complicates the design and assembling of the tunnel, while still holding that the various sections of the workpieces P in any case will not undergo the action of the same amount of sprayed liquid.
With reference to FIG. 2, it will be noted that the workpieces P are fed in the direction indicated by an arrow F substantially parallel to the two manifolds of nozzles R and thus are not evenly sprayed with liquid by the various jets G while moving through the treatment plant. More particularly, a workpiece P1 is exposed to the jets G, normally of a splayed type, and is thus subjected to a thrust which it is difficult to counteract or balance. At the same time, a workpiece P2 is not at all sprayed by jets G, whereas workpiece P3 is in a similar condition to that of workpiece P1, and so on.
In the areas between two contiguous jets G, such as that in which workpiece P2 is located, the sprays G along the entire height of the manifolds R have very little effect on the workpiece since on one hand they collide with and neutralize each other, and on the other by being located at the edges of the range of action of their respective nozzle their action is far weaker and less effective and thus they promote more formation of mist or vapor rather than having some effect on the workpiece P2 in transit. Accordingly, the workpieces P are effectively treated only at two opposite series of nozzles U (sections P1, P3 P5 . . . ), whereas at their intermediate sections (P2, P4 . . . ) they are treated to a much lesser extent or not at all treated.
This circumstance, which is bound to the design of a spray tunnel with nozzles on fixed manifolds, is responsible for transverse swinging and collisions of the workpieces P, which, in turn, frequently results in the workpieces P tending to rotate about their own axis in a random manner and to swing (pendulum effect) in the feeding direction or in a direction normal to it with frequent collisions, entanglements up to the point in which adjacent workpieces are superimposed on each other, and gluing together phenomena between two or more workpieces P, especially when they have relatively large flat surfaces, with the consequence that two glued-together workpieces are treated at part only of their outer surface.
These phenomena often result also in workpieces P being disengaged and falling off in the treatment tunnel with consequent easily imaginable serious inconveniences, such as plant stoppage, removal of fallen workpieces, repair of plant components in case of damage, replacement of workpieces, and so on. It is therefore necessary that the plant be continuously supervised by operators to avoid production waste or at least reduce it to a minimum.
Another disadvantage of conventional pre-treatment tunnels consists in that the nozzles atomize the treatment liquid when producing jets G, which results in atomization developing along the entire length of the workpiece P. This atomization, especially in the hot sections of the pre-treatment tunnel, inevitably results in random sprays of liquid as well as clouds of mist and vapor being formed inside the pre-treatment tunnel, which inevitably causes treatment liquids to be transferred from one section of the tunnel to the other with consequent contamination of the treatment liquids.
Atomization also promotes dispersion of the heart in the pre-treatment liquids which, at the very least, heats the walls (metal sheets) of the tunnel and the environment to no purpose rather than maintaining the detergent at the temperature required by the pre-treatment liquids themselves. This results in significant heat losses with consequent elevated running costs for the pre-treatment plant. Furthermore, the vapors produced by the atomization are usually polluting for the environment, and must be collected and purified before disposal. Obviously, the supplementary equipment required for separating and/or purifying the atomized drops contribute to further increased plant and running costs.